This invention relates generally to image processing and deals more specifically with an apparatus and related method for improving the gray scale resolution of a graphic made up of pixel areas on a receiving surface wherein each pixel area is comprised of a fixed number of potential dot positions printed with substantially equal sized dots.
It is generally well understood in the ink jet printing art to vary the number of dots applied to a pixel area also referred to herein as a pixel to produce shades of gray. The number of shades of gray are directly related to the number of potential dot positions that may be printed within a pixel area and one such system is disclosed by Behane in U.S. Pat. No. 3,604,846 issued Sept. 14, 1971 and entitled METHOD AND SYSTEM FOR RECONSTRUCTION OF HALF-TONE IMAGES. One drawback in generating half-tone images using a fixed number of potential dot positions printed with substantially equal sized dots, such as, for example, as described by Behane, is that only an integral number of gray scale values can be generated. Thus, an image density having a corresponding gray scale value falling between two successive integer values must be rounded upward or downward to the nearest integer and the associated pixel area is printed to produce a gray scale value corresponding to the rounded off integer. If a number of adjacent side-by-side pixels have gray scale values falling between the same two successive integer values, then those pixels also have their respective gray scale values rounded to the nearest integer each pixel having the same value. The resulting graphic is printed with the rounded off gray scale values rather than the gray scale values corresponding to an original graphic to be reproduced. It is desirable therefore to improve the gray scale resolution of printed pixel areas comprising a graphic so that the resulting printed gray scale more accurately approximates the gray scale values of the original graphic.
An increase in gray scale resolution is especially desirable in the production of large-scale, color graphics such as, for example, outdoor advertising billboards and signs or displays for use in railroad or airport terminals, shopping malls and other open public areas wherein the graphics are generated from polychromatic half-tone pixel areas created by applying relatively large volume ink drops of substantially equal size to a fixed number of potential dot positions comprising pixel areas of the receiving surface.
It has been observed that one way to effectively achieve an increase in gray scale resolution for adjacent side-by-side pixel areas having gray scale values falling between the same two successive integer values is to print each of the two adjacent pixel areas using integer values such that the average gray scale value produced by the two printed adjacent pixel areas falls between the successive integer values because the human eye tends to integrate the graphic area focused upon and sensed the average gray scale reflected from the adjacent pixels of the receiving surface.
A general aim of the invention is therefore, to provide an apparatus and related method for increasing the gray scale resolution of pixel areas of a receiving surface without increasing the number of potential dot positions that may be printed within the pixel to produce a finished graphic that is of high quality and having a gray scale value that more nearly approximates the image density of an original graphic.
These general aims are achieved in accordance with the invention, and as described in more detail hereinafter, by image density processing apparatus that increments and decrements in each of two adjacent pixel areas a predetermined dot level value so that the image density of each of the pixel areas corresponds to a dot level value falling between the same two successive integer values whereby the incremented value pixel and decremented value pixel are averaged together so that the resulting gray scale produced by the two adjacent printed pixel areas more nearly approximates the gray scale value corresponding to the image density of an original graphic measured at those corresponding pixel areas.
Other objects and advantages of the present invention will be apparent from the following written description and claims taken in conjunction with the accompanying drawings.